Life on a Meteor Ride
Artist's depiction of the Chicxulub impact crater. The total number of
objects a kilometer in diameter or larger, a size that could cause global
catastrophe upon Earth impact, is now estimated to range between 900 and
1,230.
Credit: NASA
The British molecular biologist Francis Harry Crick died on Wednesday at the
age of 88. Crick changed our understanding of life when, in 1953, he and
James Watson announced that DNA came packaged in an elegant double helix
structure. Crick reportedly claimed they had found 'the secret of life,' and
many scientists agree. The double-helix structure explained how genetic
material replicated through nitrogenous base pair bonds. Some see this as
the most important development in biology in the 20th century, and Watson
and Crick were awarded the Nobel Prize in Medicine for their discovery in
1962.
Crick was not content to sit back on his laurels after winning one of the
top prizes in science, however. He continued to study the mysteries of life,
such as the nature of consciousness, or the possibility that RNA preceded
the development of DNA. In 1973, he and the chemist Leslie Orgel published a
paper in the journal Icarus suggesting that life may have arrived on Earth
through a process called 'Directed Panspermia.'
see Great Impact Debate Part 1 * Part 2 * Part 3 * Part 4 * Part 5
The Panspermia hypothesis suggests that the seeds of life are common in the
universe and can be spread between worlds. This idea originated with the
Greek philosopher Anaxagoras, and was later promoted by the Swedish
physicist Svante Arrhenius and the British astronomer Fred Hoyle. Versions
of this hypothesis have survived to the present day, with the discovery of
proposed 'fossil structures' in the martian meteorite ALH84001.
In a related project conducted by members of NASA's Astrobiology Institute,
scientists have created primitive organic cell-like structures. They did it
in their laboratory by duplicating the harsh conditions of cold interstellar
space! Did comets carry such protocells to Earth?
'Directed Panspermia' suggests that life may be distributed by an advanced
extraterrestrial civilization. Crick and Orgel argued that DNA encapsulated
within small grains could be fired in all directions by such a civilization
in order to spread life within the universe. Their abstract in the 1973
Icarus paper reads:
"It now seems unlikely that extraterrestrial living organisms could have
reached the earth either as spores driven by the radiation pressure from
another star or as living organisms imbedded in a meteorite. As an
alternative to these nineteenth-century mechanisms, we have considered
Directed Panspermia, the theory that organisms were deliberately transmitted
to the earth by intelligent beings on another planet. We conclude that it is
possible that life reached the earth in this way, but that the scientific
evidence is inadequate at the present time to say anything about the
probability. We draw attention to the kinds of evidence that might throw
additional light on the topic."
The Miller-Urey experiment generated electric sparks -- meant to model
lightning -- in a mixture of gases thought to resemble Earth's early
atmosphere.
Credit: AccessExcellence.org
Crick and Orgel further expanded on this idea in their 1981 book, 'Life
Itself.'. They believed there was little chance that microorganisms could be
transported between planets and across interstellar distances by random
accident. But a technological civilization could direct panspermia by
stocking a spacecraft with a genetic starter kit. They suggested that a
large sample of different microorganisms with minimal nutritional needs
could survive the long journey between worlds.
Many scientists are critical of the Panspermia hypothesis, because it does
not try to answer the question of how life first originated. Instead, it
passes the responsibility on to another place and another time, offering at
best a partial solution to the question.
Crick and Orgel suggested that Directed Panspermia might help resolve some
mysteries about life's biochemistry. For instance, it could be the reason
why the biological systems of Earth are dependent on molybdenum, when the
chemically similar metals chromium and nickel are far more abundant. They
suggested that the seeds for life on Earth could have originated from a
location far richer in molybdenum.
Other scientists have noted, however, that in seawater molybdenum is more
abundant than either chromium or nickel.
Coming full circle to his groundbreaking discovery of DNA's structure, Crick
wondered, if life began in the great "primeval soup" suggested by the
Miller/Urey experiment, why there wouldn't be a multitude of genetic
materials among the different life forms. Instead, all life on Earth shares
the same basic DNA structure.
Crick and Orgel wrote in their book 'Life Itself,' "an honest man, armed
with all the knowledge available to us now, could only state that in some
sense, the origin of life appears at the moment to be almost a miracle, so
many are the conditions which would have had to have been satisfied to get
it going."